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Mistress Siana:]And calculated was exactly how Snape's look was described after Harry talked to the snake.
Why would Snape be giving Harry a shrewd and calaulating look if he knew taht he was a parseltounge... wouldnt' he be in an elevated sense of achievement or just merely smug/happy since the parseltounge thing had been reveled to the rest of the school??? What would Snape need to be calculating if he knew about it already???
I think that as Snape just found out the parseltougne thing, he was trying to either figure out what it means that harry is a parseltounge (wondering about any possible connection to Voldie/Salazar) and possibly even how Snape could use the info to his advantage???